I. Field of the Invention
The present invention relates to the field of electrodeless alkali metal vapor discharge lamps in which an optical discharge is excited by an externally-applied, high-frequency field. More particularly, this invention is directed to a lamp assembly for providing lamp excitation to produce useful spectral lines, but which eliminates the need for an excitation coil and thereby reduces substantially the size of the assembly.
II. Description of the Prior Art
Electrodeless vapor discharge lamps that produce spectral line light emission through the ionization action of electromagnetic fields upon a vaporizable alkali metal are well-known. Such vapor discharge lamp assemblies typically comprise a lamp, an excitation coil, and an oven in which the lamp and coil reside. The lamp includes a sealed bulb or envelope in which the vaporizable substance is confined along with a buffer gas. The buffer gas is commonly one of the noble gases, such as argon, neon, helium, krypton or xenon.
An alkali metal, namely rubidium, cesium, potassium, sodium, or lithium, is used as the vaporizable substance within the lamp bulb. Ionization is effected through high-frequency electromagnetic fields provided by an excitation coil which is external to and encircles the envelope of the lamp to produce a longitudinal magnetic field along the axis of the lamp bulb. As is generally understood in the art, excitation of the alkali metal vapor is occasioned by a circumferential electric field which is proportional to the time derivative of the longitudinal magnetic field. Light emission or discharge is thus maintained without the use of electrodes in the envelope. Discharge lamps of this type are comparatively simple in structure, are relatively inexpensive to build and operate, and generally have a relatively long life because of the absence of electrodes. Such lamps are efficient and stable sources of highly-resolved optical spectral line radiation.
By use of this type of vapor discharge lamp, it is possible to concentrate an optical output in a very narrow spectral band with a minimum of intensity fluctuation and noise. These lamps find utility, for example, in the operation of various high-resolution optical systems and have been used extensively in the development of commercially feasible instruments employing the principles of optical transmission monitoring of the magnetic sublevels of atoms or other quantum systems. Further, such lamps may be used as optical lamps to achieve highly-accurate control of a radio frequency signal based on the principles of atomic resonance. The signal is then used as a frequency standard.
The excitation coil which provides lamp excitation through application of high-frequency electromagnetic fields is designed to operate in the radio-frequency (RF) range and is normally wound about the longitudinal axis of a lamp bulb and at one end of the lamp bulb or envelope. This RF coil is either driven by an RF oscillator circuit or is itself part of the oscillator circuit such as the RF coil in a tank circuit of a Colpitts oscillator. While the use of an RF coil wound about the lamp envelope has proven satisfactory in operation to ionize the alkali metal vapors, there are disadvantages to its use.
An oven used in such prior art lamp assemblies is often a cylindrical shell which surrounds the lamp and RF coil and is heated by a heating element or heating coil powered by an external power supply. The heating element and oven are needed to maintain the lamp assembly at a predetermined temperature so that the alkali metal is vaporized and the vapor pressure of the vaporized alkali metal is kept at the desired level at which light emission occurs when an energizing field is applied by the RF coil. The need for an oven, however, requires that the design of the assembly be such that there is no physical distortion or damage to the RF coil and that the output of the coil is also not distorted. These demands necessitate the spacing between the oven wall and the RF coil to be sufficiently great to avoid excess loading of the coil and thus a reduction in or destruction of its effectiveness. Typically, the oven wall must be spaced from the RF coil a distance approximately equal to the radius of the coil. The end result of these conditions of construction is that the lamp assembly is made significantly larger and heavier and thus more costly than might otherwise be necessary.